Process for treating aluminum surfaces



July 25, 1961 G. NESSIM 2,993,819

PROCESS FOR TREATING ALUMINIUM SURFACES Filed April 12, 1960 INVENTOR.

'BY W5:

r* 2,993,819 1C6 Patented July 25, 1961 2,993,819 PROCESS FOR TREATING ALUMINIUM SURFACES Leon Gabriel Nessim, Geneva, Switzerland, assignor to 'tChimel S.A., Geneva, Switzerland, a Swiss corpora- Filed Apr. 12, 1960, Ser. No. 21,777 5 Claims. (Cl. 1486.27)

This invention relates to coating and more particularly to the treatment of metallic coatings.

A principal object of the present invention is to provide a process for improving the corrosion resistance of metallic coatings, particularly of aluminum.

Another object of the invention is to provide a process for improving the corrosion resistance of metallic coatings obtained by vacuum evaporating and depositing a metal upon a substrate or article.

Still another object of the invention is to provide a process for improving the corrosion resistance of vacuum-deposited aluminum coatings.

Still another object of the invention is to provide a process for improving the corrosion resistance of aluminum which has been vacuum-deposited upon steel strip.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation and order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing which is a diagrammatic, schematic view of one embodiment of the invention.

It is well known that many metallic substrates or articles which are subject to corrosive oxidation or deterioration by saline conditions or the like may be coated with aluminum to provide corrosion resistance thereto. Thin aluminum coatings are usually of a porous nature and thus usually do not provide adequate corrosion-resistant coatings. Relatively thick aluminum coatings, i.e., in excess of about 1 micron, generally provide a somewhat more durable corrosion-resistant coating. The present invention is particularly directed to improving the corrosion resistance of aluminum coatings such as those produced by vacuum-depositing processes.

The process of the present invention for rendering aluminum coatings more corrosion resistant and for imparting longer effectiveness thereto comprises subjecting an aluminum-coated metal substrate or article to hydrogen fluoride gas at an elevated temperature of between about 200 and 400 C., then passing the coated substrate through an aqueous sodium fluoride bath maintained at a temperature between about 80 and 90 C., and baking the resultant sodium aluminum fluoride coating. In one embodiment of the invent-ion the baked sodium aluminum fluoride coating is rinsed with water to remove any sodium fluoride which may be present and then dried.

Referring now to the drawing, there is shown one prefer-red embodiment of the invention wherein represents a reaction chamber into which there is introduced through conduit 12 hydrogen fluoride gas from a suitable source. The chamber 10 is provided with suitable heating means 14 such as, for example, resistance heating and the like. During operation the reaction chamber 10 is preferably heated to a temperature between about 200 and 400 C. Hydrogen fluoride gas and hydrogen which results from the reaction are removed from 2 chamber 10 through conduit 16 and suitably disposed such as by scrubbing with a liquid in which the gases possess substantial solubility. Roller seals 18 are provided at each end of the chamber '10. The pressure within the chamber is maintained at atmospheric or somewhat above atmospheric pressure during operation.

Substrate 20 is guided by rollers 22 through a concentrated aqueous sodium fluoride bath 24 contained within vessel 26 preferably heated to a temperature between about to C. by suitable heating means 28, e.g., resistance heating means. After passing through the sodium fluoride bath, the resultant coating comprising sodium aluminum fluoride is baked by heating means 30 here shown as being heat lamps. In order to remove any sodium fluoride, the baked coating is preferably rinsed with water and then dried.

In carrying out the preferred process of the .present invention, a metallic substrate or sheet material such as steel strip having at least one side coated with aluminum is introduced into reaction chamber '10. In one embodiment, the aluminum coating is produced by vacuum evaporation and deposition. It is obvious that aluminumcoated steel strip may be fed directly from a vacuum coating system into the reaction chamber 10 so as to provide for one continuous process of vacuum aluminizing and treatment of the aluminum coating to improve the corrosion resistance thereof. The aluminum-coated steel strip while passing through chamber 10 is subjected to elevated temperatures preferably on the order of between about 200 and 400 C. and to suflicient hydrogen fluoride gas. The aluminum or at least the surface portions of the aluminum coating react with the hydrogen fluonide to produce aluminum fluoride and hydrogen. This reaction is as follows:

The pressure within the chamber 10 is maintained at atmospheric or slightly above. Some hydrogen gas leaks out through the seals and some is removed from the chamber 10 with hydrogen fluoride gas via conduit 16 and suitably disposed of or the gases separated and recovered.

Upon removal from chamber 10, the steel strip having thereon a coating comprising aluminum fluoride is cooled such as by running through an air gap prior to being immersed in a concentrated aqueous solution of sodium fluoride preferably maintained at a temperature between about 80 and 90 C. Contact with the aqueous sodium fluoride bat-h results in the conversion of this aluminum fluoride coating to sodium aluminum fluoride. This reaction is as follows:

The resultant sodium aluminum fluoride coating is then baked, for example, at a temperature on the order of about 200 C. To remove any sodium fluoride that may have adhered to the coating, it may be rinsed with water and then dried.

There is thus produced, according to the present invention, a steel strip having a hard flexible, adherent sodium aluminum fluoride coating which is extremely resistant to even the most severe corrosive forces for long periods of time.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What I claim is:

1. A process for treating an aluminum coated metal article surface which comprises subjecting the aluminum coating of said article to hydrogen fluoride gas at a temperature between about 200 and 400 C., contacting the resultant aluminum fluoride surface with a solution of sodium fluoride maintained at a temperature between about 80 and 90 C., and then baking the resultant sodium aluminum fluoride coating at a temperature of about 200 C.

2. A process for treating aluminum-coated steel strip which comprises passing said strip through an atmosphere comprising hydrogen fluoride gas maintained at a temperature between about 200 and 400 C. for a period of time suflicient to convert the surface of said coating to aluminum fluoride, then passing said strip through a solution comprising sodium fluoride maintained at a temperature between about 80 and 90 C., and thereafter sistance to vacuum-alumiuized steel strip which comprises subjecting the vacuum-deposited aluminum coating to an atmosphere comprising hydrogen fluoride gas heated to a temperature between about 200 and 400 C., contacting the resultant coating comprising aluminum fluoride with a concentrated aqueous solution of sodium fluoride heated to a temperature between about and C., and thereafter baking the resultant coating comprising sodium aluminum fluoride at a temperature of about 200 C.

5. A process according to claim 4, wherein the baked coating comprising sodium aluminum fluoride is rinsed with water and then dried.

References Cited in the file of this patent UNITED STATES PATENTS 2,092,033 Stroup Sept. 7, 1937 2,092,034 Stroup Sept. 7, 1937 2,382,432 McManus et al. Aug. 14, 1945 2,824,819 Smith Feb. 25, 1958 ,885,316 Milliken May 5, 1959 

1. A PROCESS FOR TREATING AN ALUMINUM COATED METAL ARTICLE SURFACE WHICH COMPRISES SUBJECTING THE ALUMINUM COATING OF SAID ARTICLE TO HYDROGEN FLUORIDE GAS AT A TEMPERATURE BETWEEN ABOUT 200* AND 400*C., CONTACTING THE RESULTANT ALUMINUM FLUORIDE SURFACE WITH A SOLUTION OF SODIUM FLUORIDE MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 80* AND 90*C., AND THEN BAKING THE RESULTANT SODIUM ALUMINUM FLUORIDE COATING AT A TEMPERATURE OF ABOUT 200*C. 